Reference genome

The first printout of the human reference genome presented as a series of books, displayed at the Wellcome Collection, London

A reference genome (also known as a reference assembly) is a digital nucleic acid sequence database, assembled by scientists as a representative example of a species' set of genes. As they are often assembled from the sequencing of DNA from a number of donors, reference genomes do not accurately represent the set of genes of any single person. Instead a reference provides a haploid mosaic of different DNA sequences from each donor. For example, GRCh37, the Genome Reference Consortium human genome (build 37) is derived from thirteen anonymous volunteers from Buffalo, New York.[1][2][3] The ABO blood group system differs among humans, but the human reference genome contains only an O allele (although the other alleles are annotated).[4]

As the cost of DNA sequencing falls, and new full genome sequencing technologies emerge, more genome sequences continue to be generated. Reference genomes are typically used as a guide on which new genomes are built, enabling them to be assembled much more quickly and cheaply than the initial Human Genome Project. Most individuals with their entire genome sequenced, such as James D. Watson, had their genome assembled in this manner.[5][6] For much of a genome, the reference provides a good approximation of the DNA of any single individual. But in regions with high allelic diversity, such as the major histocompatibility complex in humans and the major urinary proteins of mice, the reference genome may differ significantly from other individuals.[7][8][9] Comparison between the reference (build 36) and Watson's genome revealed 3.3 million single nucleotide polymorphism differences, while about 1.4 percent of his DNA could not be matched to the reference genome at all.[2][5] For regions where there is known to be large scale variation, sets of alternate loci are assembled alongside the reference locus.

Contents

A simple way to measure genome length is to count the number of base pairs in the assembly.[11]

The golden path is an alternative measure of length that omits redundant regions such as haplotypes and pseudoautosomal regions.[12][13] It is usually constructed by layering sequencing information over a physical map to combine scaffold information. It is a 'best estimate' of what the genome will look like and typically includes gaps, making it longer than the typical base pair assembly.[14]

The previous human reference genome (GRCh37) was the nineteenth version. This build contained around 250 gaps, whereas the first version had roughly 150,000 gaps.[1] The GRCh38 assembly saw the closure or reduction of more than 100 gaps. Nanopore sequencing has seen the closure of 12 gaps in the GRCh38 reference assembly through the use of ultra-long reads.[16]

^Donors were recruited by advertisement in The Buffalo News, on Sunday, March 23, 1997. The first ten male and ten female volunteers were invited to make an appointment with the project's genetic counselors and donate blood from which DNA was extracted. As a result of how the DNA samples were processed, about 80 percent of the reference genome came from eight people and one male, designated RP11, accounts for 66 percent of the total.